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Pin Fin Heat Sinks: A Sharper Way to Keep Medical Electronics Cool

Mon, 10/05/2009 - 6:19am
Barry Dagan
Keeping electronics cool is critical to ensuring their long life and reliability. Utilizing heat sinks can help achieve this, but determining the proper type and configuration can be a challenge. This article reviews pin fin heat sinks, material options, sparse vs. dense pin configurations, and more factors in determining what is needed for an application.


Barry DaganP. E. is the chief technology officer of Cool Innovations Inc.Dagan has more than 25 years of experience in thermal management design and holds multiple patents. He can be reached at 905-760-1992 or barryd@coolinnovations.com.

Electronic medical systems have become substantially more sophisticated and powerful in recent years. As a result, the majority of these systems contain large arrays of power hungry electronic components that require high-performance heat sinks to prevent them from overheating and being damaged or destroyed.

Copper pin fin heat sinks are suitable for extreme cooling needs and for scenarios that require rapid heat spreading.
The electronic components used in medical equipment perform a variety of functions and, consequently, their levels of power dissipation (and heat generation) vary widely. They range from small semiconductors that dissipate heat loads as low as one watt to high-power laser and radio-frequency devices that dissipate heat loads approaching 1,000 watts.

Although electronic medical systems differ in many aspects, the majority of medical applications share a common need for high reliability and performance. At the same time, these systems require small size and quiet operation. Heat sinks play an important role in helping system designers meet all these goals.

By efficiently pulling heat away from the power dissipating components, heat sinks ensure component reliability. Meanwhile, the choice of a proper heat sink can also eliminate or reduce the need for fans, which can degrade system reliability and introduce audible noise during system operation. In addition, high-performance heat sinks can efficiently cool components without taking up much space.

One of the best-performing heat sink technologies available today is the pin fin technology. Pin fin heat sinks efficiently cool components; can be adapted to a range of component sizes, power levels, and air speeds; and are fabricated in a highly consistent manufacturing process. Because of these qualities, pin fin heat sinks are especially suitable for a wide array of medical applications. Moreover, a recent innovation in the technology–splayed pin fin heat sinks–provides even higher levels of cooling performance in a smaller space.

What's a Pin Fin?
Absolute customization allows for the maximization of heat sink surface area and eliminates bypass air.
In a typical heat sink, such as an extrusion, the fins are rectangular-shaped pieces of metal that rise from a metal base. The fins are positioned perpendicular to the metal base, but run parallel to one another. In this style of heat sink, the term "fin" seems self explanatory since it's a flat object that juts up much the way a dorsal fin rises from the body of a dolphin.

However, the meaning of the term "pin fin" seems less obvious. In this case, the fin is a cylindrical protuberance unlike most of the fins associated with nature or man-made objects. So, a "pin fin heat sink" is a heat sink comprised of a flat base with an array of round pins that are embedded into the base.

Pin fin heat sinks are available in copper and aluminum variations. They come in a number of different pin densities, which are suitable for different airspeed environments. These heat sinks also vary by size. They are available in footprints ranging from 0.27" × 0.27" to 10" × 10" and with overall heights ranging from 0.15" up to 2.0". These different sizes accommodate a wide array of applications and devices, ranging from miniature semiconductors to large power devices.

The majority of pin fin heat sinks available today are manufactured via the cold forging technology. Cold forging is well suited to medical applications as it is inherently a highly consistent manufacturing technology. Forged pin fins do not contain any cavities, air bubbles, or impurities within their interior or exterior. (Any of those faults would potentially compromise the mechanical integrity or thermal performance of a heat sink.)

In terms of performance, pin fin heat sinks are extremely effective as they feature a low thermal resistance for a given volume. The cooling capability provided by pin fins stem from the round geometry of the pins, their omnidirectional configuration, and their use of materials that are highly conductive of heat.

The smooth round pins reduce resistance to incoming airstreams and enhance air turbulence between the pins. The omnidirectional structure maximizes the entrance of fresh air into the pin array from every possible direction and simultaneously allows the hot air to exhaust from the pin array in every possible direction. The use of highly conductive materials further reduces the thermal resistance of the heat sinks.

Selecting Heat Sinks for Medical Equipment
When choosing a heat sink, there are three main factors that designers should consider to ensure that the heat sink is optimized for their application. These factors are air speed, heat sink geometry, and the choice of metals. These issues are discussed here with respect to pin fin heat sinks, but the same concepts are applicable to other heat sink technologies as well.

Pin Density Depends on Air Speed
Impingement cooling provides up to a 20% performance premium over cooling from the side and ensures even air distribution along the surface of the heat sink.
A common misconception shared by many engineers is that heat sink performance is strictly a function of the heat sink's surface area. The perception is that more surface area means better cooling. This thinking comes into play when designers specify pin density, the spacing between pins for a given sized heat sink. Faced with different values of pin density, designers often simply specify the highest density because it offers greater surface area.

But the assumption that more surface area is better is only true when a heat sink is being placed in a high-airspeed environment. In that situation, approaching air streams are strong enough to penetrate through tight pin arrays. However, in the majority of airspeed environments–where air flows at a few hundred linear feet per minute (LFM) or less–dense heat sinks are not efficient. That's because approaching airspeeds are not strong enough to penetrate a dense array of pin fins, so the greater surface area of the pins does not produce a low thermal resistance from the heat sink to the ambient environment. In fact, a heat sink with high pin density may even act as a big block to air flow in low-airspeed environments.

Consequently, one of the key criteria in selecting pin fin heat sinks is matching the pin density to the available airspeed. For heat sinks that are placed in high-airspeed environments–400 LFM or greater–densely configured heat sinks are appropriate. Such heat sinks would also be appropriate with impingement cooling where a fan is placed directly on top of the heat sink.

For moderate-airspeed environments in the range of 200 to 400 LFM, moderately configured heat sinks that balance surface area and spacing are recommended. Finally, for natural convection cooling, where airspeed approaches 0 LFM, and limited-airspeed environments, where airflow is in the range of 0 to 200 LFM, sparsely configured heat sinks are recommended.

It Pays to Customize
The geometry of pin fin heat sinks can be easily customized to meet specific application requirements. In terms of footprint, the base of the heat sink can be made rectangular, round, or any other odd shape as individual pins can be eliminated without any effect on the remaining pins. In terms of height, any desired height can be achieved within the given minimum and maximum specifications for pin height.

When standard heat sinks do not work in the application, custom heat sinks are recommended as the first option before resorting to fan sinks or moving from aluminum to copper. That's because it's relatively inexpensive to customize pin fin heat sinks, yet doing so can greatly improve performance.

For medical applications that require reliability, custom heat sinks can improve thermal performance in two ways. A customized heat sink geometry can improve heat sink performance (i.e., reduce thermal resistance) by making the best use of the space through an increase in total heat sink surface area. In addition, the customized geometry can eliminate bypass air, which is the air that is forced to go over or around the heat sink.

Copper or Aluminum?
The splayed pin fin structure enables air to enter and exit the pin array in a more efficient fashion and, therefore, offers a substantial cooling premium.
For the majority of cooling scenarios, aluminum heat sinks are preferable over copper heat sinks. Copper heat sinks provide only slightly better cooling performance than identically structured aluminum heat sinks. However, the price for that modest improvement is a heavier, more-expensive heat sink.

In terms of cost, copper heat sinks are generally offered at a 50% to 100% premium versus aluminum. As for weight, copper is 3.2 times heavier then aluminum. For these reasons, copper heat sinks are not often recommended solely for the thermal resistance premium they provide.

However, copper heat sinks do offer a unique and highly valuable characteristic–their outstanding heat-spreading capability. Because copper has a high thermal conductivity–twice that of aluminum–copper heat sinks are highly suitable for cooling devices that are very small with a concentrated source of heat.

In these instances, the heat sink must be able to spread the heat quickly along its base in order for it to efficiently cool the power-dissipating device. Otherwise, the areas of the heat sink far away from the device will not be able to provide any cooling power. Such is typically the case with aluminum heat sinks when the heat source is concentrated.

So, even though copper heat sinks only provide a modest improvement in thermal resistance that generally ranges from 5% to 10% for most applications, the improvement in thermal performance is more significant for small, hot-running devices. In such cooling applications, the use of copper typically reduces the heat sink's thermal resistance by 15% to 25% versus an aluminum heat sink.

Looking Forward
As integrated circuit (IC) technology continues to evolve, the power levels dissipated by cutting edge ICs continue to rise. Over time, these components are forcing design engineers to look for more-effective heat sink technologies that can provide more cooling in a smaller space. Splayed pin fin heat sinks represent one such breakthrough technology.

Splayed pin fin heat sinks exhibit the same structural properties as standard pin fin heat sinks, but with pins positioned at a different angle. Unlike standard pin fins, splayed pin fin heat sinks feature an array of pins that are bent outward gradually. By bending the pins, the distance between them is increased substantially. As a result, weak incoming airstreams are able to penetrate the pin array. At low airspeeds or with natural convection, splayed pin fin heat sinks can provide up to a 30% improvement in thermal performance versus standard pin fins.

As an example of the cooling premium generated by splayed pin fins, consider a 2.0" × 2.0" × 1.1" splayed copper pin fin heat sink with 225 pins. This heat sink exhibits a thermal resistance of 4.05°C/W in natural convection. When compared to a similarly sized, identically structured standard copper pin fin heat sink that offers a thermal resistance of 4.86°C/W, also in natural convection, the splayed pin fin provides a 20% performance premium.

Conclusion
Designers of electronic medical equipment can take advantage of the high performance afforded by pin fin heat sinks to meet a variety of design goals including small product size and reliability. Following the guidelines outlined in this article, designers can ensure that the heat sinks they specify are optimized for their applications.

Online
For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or Cool Innovations Inc. at www.coolinnovations.com.
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